1. Field of the Invention
The present invention relates to optical elements and a method of manufacturing the optical elements.
2. Description of the Related Art
Recently, organic electroluminescent (EL) display apparatus employing organic light emitting diodes (hereinafter referred to as OLED) as luminous elements are attracting much attention as display apparatus to replace CRTs and LCDs.
Holes and electrons are injected to a luminescent layer from an anode formed on a glass substrate and a cathode provided above the anode, respectively. Then, these holes and electrons recombine with each other so as to produce excitons. And in the process of radiation deactivation of the excitons, light emanates from the luminescent layer and the organic EL element thus emits the light. It is to be noted here that a hole transport layer is provided between the anode and the luminescent layer whereas an electron transport layer is provided between the cathode and the luminescent layer.
As a problem, the electron transport layer and luminescent layer in the organic EL elements structured as above are liable to be affected by impurities such as water molecules and oxygen molecules, and the deterioration thereof with time is generally conspicuous compared to LCDs or the like.
Moreover, it is difficult to stably inject electrons into the luminescent layer, so that there is a problem where variation in luminance is caused. In order to inject the electrons stably, metal with a low work function is used as cathode material. Moreover, the cathode requires such measures as lowering resistance, reducing whiskers and hillocks or eliminating electromigration or stressmigration.